It is known to etch defined features, for example trenches, combs, tongues, flexural beams, or the like, anisotropically with low to medium selectivity into silicon substrates that are preferably utilized with the semiconductor technique. The individual features to be etched in are usually defined by way of etching masks applied onto the silicon substrate, via so-called masking layers, for example a photoresist layer. In the anisotropic etching technique it is necessary to arrive at exactly laterally defined recess in the silicon. These recesses, penetrating in the depth direction, must possess lateral boundaries which are as accurately perpendicular as possible. The edges of the masking layers which cover those silicon substrate regions that are not to be etched must not be underetched, so as to maximize the lateral accuracy of the feature transfer from the mask into the silicon. This results in the need to have etching proceed only on the nature floor, and not on the previously created sidewalls of the features. It is proposed in German Patent 42 41 045 to perform the etching of profiles into silicon substrates using a method which alternatingly provides plasma polymer deposition and plasma etching steps. In this context, deposition and etching steps are performed in a chemical context based exclusively on fluorine compounds; during the inherently isotropic etching steps, forward advancement of the sidewall polymer film applied during the previous deposition steps already effectively passivates the freshly exposed portions of the silicon sidewall, so that the inherently isotropic etching step becomes locally highly anisotropic. This technique of local anisotropy by way of forward advancement of a sidewall film allows relatively wide etching steps at very high speed without etching into the sidewall, which consequently exhibits only minor roughness. In general, the performance of this plasma etching, when it is done with microwave excitation (propagation ion etching or PIE), does not create any appreciable wall roughess. This process does, however, create serious problems in a so-called inductively coupled system with high-frequency plasma excitation (ICP=inductively coupled plasma). With this, a pronounced recess becomes etched into the silicon directly beneath the edge of the photoresist mask. This recess is an effect of the inductive excitation, which is associated with magnetic and electric fields in the region of the substrate, for example a silicon wafer, and appears with greater or lesser severity on ICP systems of various designs. Inductively coupled plasma systems are playing an increasingly important role because of their robustness and versatility, and are inherently well suited for the process described above. A factor playing a major role in the formation of the etching recesses is the fact that the transition region between photoresist mask and silicon represents a discontinuity in conjunction with electric fields of the plasma source, and is exposed to a greater ion bombardment than the lower portion of the sidewall. In addition, the mechanism of the advancing sidewall film is not yet completely effective at the mask edge, and passivation of the sidewall there is thus weaker. This results in the so-called underetching, so that the etched-in silicon features no longer exhibit the necessary accuracy and critical dimensions.